Fizeau experiment
= Fizeau experiment
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= Fizeau moving-water experiment
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In 1851 <Hippolyte Fizeau> detected a change in <light> propagation through moving <water> using <interference fringes>. Sunlight entered a telescope through a partially reflecting mirror; two slits selected beams passing through parallel glass tubes, each $1.487\,\mathrm m$ long with $5.3\,\mathrm{mm}$ internal diameter. A second telescope and focal mirror exchanged the tubes on return, and the beams recombined at a graduated eyepiece. Compressed air drove <water> between reservoirs, producing opposite flows; reversing both flows reversed the <interference fringe> displacement. The double transit suppressed static differences in <temperature> and <pressure>; filling only one tube with <water> tested this compensation. Separate supports reduced mechanical disturbance. <Hippolyte Fizeau> estimated flow <speed> from <volumetric flow rate> divided by tube cross section.<The Hypotheses Relating to the Luminous Aether by Hippolyte Fizeau>[1851 report, apparatus and compensation paragraphs; flow production, independent supports and fringe observations; scanned translation pp. 569–571]{ref}
The <Fizeau experiment> turns a tiny change in <phase velocity> into a visible fraction of an <interference fringe>. The <Fizeau experiment path-length convention> below explains why both tube passages contribute with the same sign for one beam and the opposite sign for the other. This makes the apparatus sensitive to the part of the <phase difference> that reverses with the flow, while rejecting disturbances shared by both beams.